1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copying machine, a laser printer and a facsimile apparatus, or more particularly, to an improvement of a transfer unit employing a roller transfer method for transferring a toner image formed on the surface of a photoreceptor drum onto a sheet of paper.
2. Description of the Prior Art
In a transfer unit of a roller transfer method which is widely used in an image forming apparatus such as a copying machine, a conductive transfer roller is rotatively in contact with the surface of a photoreceptor drum carrying a toner image on its surface and rotating in one direction, so that a copy sheet is pressed against the toner on the drum surface. Then, a voltage reverse to that of the toner is applied to the transfer roller to transfer the toner image formed on the drum surface onto the sheet.
In a conventional transfer unit employing the roller transfer method, however, since the transfer roller is directly in contact with the drum surface when no sheet is present therebetween, residual toner on the drum surface adheres to the periphery of the transfer roller. Therefore, in the conventional transfer unit, a means is required for cleaning the transfer roller. Further, when the peripheral surface of the transfer roller is not in contact with the drum surface at a uniform and appropriate pressure, there is a non-uniformity in the image transferred to the sheet.
To solve such problems, a non-contact roller transfer method as shown in FIGS. 1 and 2 has been developed. In the transfer unit shown in these figures, a gap maintaining roller 33 having a diameter greater than that of a transfer roller 31 is attached to each end of a roller shaft 32 to which the transfer roller 31 is attached, and the peripheral surface of the gap maintaining roller 33 is in contact with the surface of a photoreceptor drum 34 so that a predetermined gap C is left between the peripheral surface of the transfer roller 31 and the drum surface.
With this arrangement, since the transfer roller 31 faces the drum 34 without being in contact therewith, the residual toner does not adhere to the peripheral surface of the transfer roller 31 and no cleaning means is necessary. Further, since the gap C between the drum surface and the peripheral surface of the transfer roller 31 is always maintained uniform by the gap maintaining roller 33, the generation of the image non-uniformity is restrained.
The arrangement of the transfer unit will be described in detail. Each of the front and rear ends of the roller shaft 32 to which the transfer roller 31 and the gap maintaining roller 33 are attached are rotatably supported by a bearing member 36. The roller shaft 32 is arranged to be parallel to the shaft of the photoreceptor drum 34. The bearing members 36 are each fixed by a screw 38 to an bearing attacher 37 arranged along the roller shaft 32. The bearing attacher 37 is supported to be swingable upward and downward by a shaft 39 arranged between side plates provided at the front and rear of the machine body. The bearing attacher 37 is pushed up by a spring 40 provided between the bearing attacher 37 and the base of the machine body. By this pushing force, the peripheral surfaces of the front and rear gap maintaining rollers 33 are brought into contact with the surface of the photoreceptor drum 34.
In the upstream side of the sheet conveying path in the vicinity of the transfer roller 31, a pre-transfer guide 35 is arranged for guiding the sheet to the drum 34. The pre-transfer guide 35 has a guide plate 35a being longer along the width of the machine body and ascending from the upstream side to the downstream side of the sheet conveying path. A bracket 35b provided to each end of the guide plate 35a is supported to be swingable upward and downward by a shaft 41 arranged between the front and rear side plates of the machine body, and is pushed up by a spring 42 provided between the bracket 35b and the base of the machine body.
At the bearing member 36, another bearing portion 36b is formed at a position closer to the front end than a transfer roller bearing portion 36a. Between the front and rear bearing portions 36b, a small-diameter pre-transfer roller 43 is arranged. The upper surface of the guide plate 35a of the transfer guide 35 is substantially in contact with the pretransfer roller 43.
A sheet conveyed to the upper surface of the pre-transfer guide 35 is brought into contact with the drum surface with its conveying speed being maintained constant by the pre-transfer roller 43, and advances into the gap C. At this time, the sheet adheres to the surface of the drum 34 mainly by the resiliency of the sheet itself. When the sheet passes through the gap C between the transfer roller 31 and the photoreceptor drum 34, a voltage is applied to the transfer roller 31, so that the toner image is transferred from the drum 34 to the sheet.
In transfer units employing the non-contact type roller transfer method, the transfer efficiency varies according to the distance between the transfer roller 31 to which the voltage is applied and the surface of the photoreceptor drum 34 which carries a toner image thereon. The distance between the transfer roller 31 and the photoreceptor drum 34 is determined in consideration of this efficiency, and the applied voltage is set to be optimum in accordance therewith. In addition, the diameters of the front and rear gap maintaining rollers 33 are the same, and the distance between the transfer roller 31 and the drum 34 are uniform along the width of the machine body, i.e. along the length of the roller shaft 32.
The toner transfer efficiency further depends on how closely the sheet and the photoreceptor drum 34 are in contact. For this reason, in order that the force which presses the sheet against the photoreceptor drum 34 is uniform along the width of the machine body, the pre-transfer guide 35 is arranged so that the distance between the upper end of the guide plate 35a and the drum 34 is uniform. With this arrangement, the toner image is transferred without any non-uniformity along the width of the machine body.
However, in the transfer unit of the above structure, since the roller shaft 32 is supported at its both ends by the bearing attacher 37 supported by the shaft 39 arranged between the front and rear side plates of the machine body, when the base or the frame of the machine body is distorted, the roller shaft 32 may not be parallel to the shaft of the photoreceptor drum 34 any more. When the roller shaft 32 is not parallel to the shaft of the photoreceptor drum 34, only one of the gap maintaining rollers 33, which are attached in the vicinity of the both ends of the roller shaft 32, is in contact with the drum 34, so that it is impossible to maintain the gap C between the transfer roller 31 and the photoreceptor drum 34 to be uniform along the shaft of the drum 34. When the gap C is not uniform, the transfer efficiency is not uniform, so that there is a non-uniformity in the transferred image.
In addition, since the pre-transfer guide 35 is supported by the shaft 41 arranged between the side plates of the machine body, when the base or the frame of the machine body is distorted, the distance between the upper end of the guide plate 35a and the photoreceptor drum 34 is not uniform. Since the sheet is in close contact with the drum surface mainly because of its own resiliency as described previously, the closeness of the contact of the sheet with the drum surface varies with the variation in the distance between the guide plate 35a and the drum 34. Since the toner transfer efficiency also varies according to the closeness, the position shift of the pretransfer guide 35 makes uniform image transfer impossible.
The distortion of the machine body which exerts such bad influences may be caused not only by a great external shock but also by the unevenness of the floor on which the machine is placed. Therefore, it is desirable for the transfer roller and the pretransfer guide to be located always at uniform and fixed distances from the photoreceptor drum whether the machine body is distorted or not.